A metering pump of the above type is known in which the coupling assembly has a transmission lever which engages on the clamping lever. The clamping lever is mounted radially on the rotary slide valve. The transmission lever is constructed with a variable length. For this purpose, it is subdivided and provided with appropriate threaded parts. The clamping lever engages on the rotary slide valve by means of an adjusting spring so that the radial position of the clamping lever relative to the rotary slide valve and its openings cannot be altered. The length of the transmission lever can only be changed when the metering pump is stationary. Altering the length of the transmission lever changes the position of the rotational angle through which the clamping lever and the rotary slide valve pass during the forward-and-backward rotational movement, while the size of this angle, generally 90.degree., remains roughly the same upon such a displacement. Such a displacement in the position of the rotational angle, which can only be performed when the metering pump is stationary, is awkward and a disadvantage. The metering pump must be stopped for every single adjustment or readjustment respectively, i.e. the filling process must be interrupted. The housing for the metering pump must be opened at the appropriate point to enable access to the transmission lever. The length can then be modified. Only when the filling process is restarted can it be seen whether or not the adjustment has achieved the desired result. If this is not the case, the entire adjusting procedure must be repeated. Should the viscosity of the filling change during the filling process, something that can happen, then a readjustment is practically impossible with the known metering pump. However, in many cases such a readjustment is highly desirable or even essential with filling processes involving difficult-to-handle fillings. As the rotary slide valve functions, in order to link the container to be filled with the piston/cylinder unit upon the intake (suction) stroke and to block this link upon the expel (eject) stroke as well as to link the piston/cylinder unit with the discharge line, there will always be such relationships whereby the chain of filling at the end of the discharge line, i.e. already on both sides of the rotary slide valve, must tear off in order to supply the respective portion. It is known to suck back filling from the discharge line at the start of the intake stroke of the piston/cylinder unit, whereby said filling then, facing the opposite way to the filling to be delivered being acted upon by gravity, moves upwards or backwards respectively in the discharge line. These two opposed movements are intended to achieve a clean breakage of the filling after each portion. According to the type of filling, this "suck-back" (return) effect has to be set differently and reset in many cases so that just such a clean break in the filling is achieved and so that traces of filling land, for example, on the sealing surfaces of the packaging container for the filling where they might interfere with the sealing. As with such metering pumps the filling volume must also be set, there is the added difficulty that when adjusting the filling volume the relationships at the break-off point of the filling chain will once again change. A relative setting is necessary here. It can be seen that these possibilities for adjustment are unsatisfactory in the case of the known metering pump.